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Inorganic Chemistry > Advanced Transition Metals > Flashcards

Advanced Transition Metals Flashcards

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1
Q

What orbitals in an octahedral complex have the highest energy

A

Orbitals along the axis

Dx^2-y^2
Dz^2

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2
Q

Order of orbitals in a square planar complex

A

Dx^2-y^2
Dxy
Dz^2
Dxz dyz

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3
Q

Tetrahedral energy level order

A

Dxy dxz dyz

Dz^2 dx^2-y^2

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4
Q

Describe the MOs of an octahedral complex with six sigma donor ligands

A

6 ligand orbitals and 9 metal orbitals

6 bonding 6 anti bonding and 3 non bonding

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5
Q

The optimum number of valence electrons

A

Between 12 and 18 valence electrons

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6
Q

Rules of electron counting:

1) metal valence electrons
2) for ligands
3) oxidation state

A

1) group number is the number of valence electrons of the metal

2) L-type are 2 electron donors
X-type are 1 electron donors
Z-type are 0 electron donors (Lewis acids)

3) oxidation state of metal is the number of x type ligands

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7
Q

Describe transmetallation

A

Synthesis method for metal alkyl/aryls

Bigger difference in electronegativity increases rate of reaction but decreases the amount of selectivity

Stereochemistry, temperature and concentration also affect reaction

Reactants include RLi RMgX ZnR2 AlR3

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8
Q

Describe electrophilic attack as a synthesis route for metal alkys

A

R+ reagent
Nucleophilic metal compound required

Eg Na[Mn(CO)5] + MeI goes to
[Mn(Me)(CO)5] + NaI

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9
Q

Describe oxidative addition as a synthesis route for metal alkyls

A

Often observed in d8 sq planar complexes as they are most vulnerable along the perpendicular to the plane of the molecule

Literally just add things in e.g. MeI
They will be cis to each other

Normally 16 e- but can be 18 if prior de coordination occurs

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10
Q

Describe 2 methods of synthesis for fisher carbenes

A

1) nucleophilic attack at carbonyl ligand

Very versatile

A range of heteroatoms can be introduced

2) electrophilic abstraction from a metal alkyl complex

Using Me3SiCl

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11
Q

2 methods of synthesis for schronk metal complexes

A

1) alpha hydrogen elimination from a dialkyl precursor

The bulky ligand means the process is driven by steric hindrance

2) alpha deprotonation of a metal methyl complex

Use NaOMe

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12
Q

If a metal carbonyl has a lot of backbonding, where is it susceptible to attack

A

Electrophilic attack at oxygen

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13
Q

If a metal carbonyl has poor backbonding where is it susceptible to attack?

A

Nucleophilic attack at carbon

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14
Q

What type of ligands are alkenes

A

L-type

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15
Q

2 effects of backbonding to an alkene

A

Increases carbon carbon bond length

Reduces angles around carbon centres as sp3 contribution increases

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16
Q

Is a metal-alkenyl or metallacyclo structure more reactive?

A

Alkene is d+ and therefore is susceptible to nucleophilic attack

Increased back bonding in the cyclic compound reduces the charge so the metallocyloalkane is less reactive

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17
Q

A metal carbon sp3 bond describes what type of compound

A

Metal alkyl including cycles

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18
Q

Metal carbon sp2 bonds are found in what 3 types:

A

Metal-aryl
Metal-alkenyl
Metal-acyl

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19
Q

Metal carbon(sp) bonds are found in which 3?

A

Metal carbonyl
Metal alkynyl
Metal isonytryls

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20
Q

Oxidative addition as a main reaction:

A

Oxidation number, coordination number and VE count all increase by 2

Metals with low oxidation states

Can have concerted, stepwise, radical or ionic mechanisms

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21
Q

Reductive elimination

A

Oxidation state, coordination number and VE count all decrease (-2)

Metals in medium or high oxidation states

Non polar 3 centre transition state

22
Q

Sigma metathesis

A

Typical of early d-block elements in high oxidation states

No change in oxidation state

4 membered cyclic transition state

Concerted process

23
Q

Ligand substitution

A

D electron count, coordination and d electron count remain unchanged

Can be associative or dissociative

24
Q

Migratory insertion

1,2 and 1,1 insertion

A

No change in oxidation state

Ligands must be cos

1,2 insertion: an unsaturated L type ligand is inserted into a sigma M-X bond after the unsaturated ligand has associated to the metal

1,1 insertion: typical of a carbonyl complex, an x ligand migrates to a coordinated co to form an acyl, leaving one vacant site

25
Q

Hydride elimination reactions

A

Reverse of insertion

No change in oxidation state

Alkene often eliminated

26
Q

Nucleophilic attack of coordinated ligand

A

Unsaturated organic compounds are activated towards nucleophiles when coordinated to electron deficient metals

Causes elimination of alkene

27
Q

Electrophilic abstraction

A

Alkyl or hydride ligands might be abstracted by strong electrophiles such as B(C6F5)3

28
Q

Coordinated alkane spectra:

Proton NMR

A

Do not differ drastically

Dependant in metal, oxidation state and other ligands

29
Q

Coordinated alkane spectra

C NMR:

A

Both positive and negative chemical shifts

For early metals the signals are more downfield due to deshielding (the opposite is true for late transition metals)

30
Q

Coordinated arene C NMR

A

For metal aryls the sigma bound ipso carbon tends to be more deshielded than the corresponding arene

Arene substituents also affect chemical shift, depending on donating and withdrawing effects

31
Q

Increasing kinetic stability of M-C bonds

A

Thermodynamically stable but there are many reaction pathways so kinetically unstable

Coordinatively saturated complexes with no Kabila ligands are more stable

Increasing steric hindrance with bulky ligands also increases stability UNLESS PREVENTING ELIMINATION

32
Q

Beta hydride elimination reactions and 3 things you need:

A

Mainly metal alkyls

Driving force is the formation of a stronger M-H bond and generation of an alkene (reduces the unsaturation of the metal)

Need:

B-hydrogens
Vacant sites
M-c-c-h syn-coplanar arrangement possible

33
Q

How can beta hydride elimination reactions be avoided:

A

Using ligands with no b-hydrogens

Using metals with no empty coordination sites

Small metallacycles show higher stability as there is no syn-coplanar orientation. The smaller the cycle the more stable it is but they still react when heated

Double bonds to bridgehead carbons are unfavourable

34
Q

Alpha hydride elimination reactions

A

Observed when B-H elimination is not possible

Mo and Ta are most likely to react in this way

Alpha hydrogens must be available

Free coordination site cis to CH3

Product is very reactive and will proceed to react with any nucleophile

All elimination favoured more as steric hindrance around metal increases

35
Q

Benzyne formation

A

Not common due to instability of products

CH bond activation process

High reaction temps and increasing steric hindrance around a metal

Can go on to form many organic compounds with stereochemical control due to insertion step

Alkyne lumo is very low therefore acts as an electrophile

36
Q

Reductive elimination

A

Reverse of oxidative addition

Two groups in a cis arrangement

More common for alkyl and square planar complexes

Prevented by strongly chelating ligands and restrictive geometries

Common for biaryl decomposition, where the reactant molecule can be free or bound to the metal centre

37
Q

Cross coupling catalysts:

A

1) classics [Pd(PPh3)4] , [Pd2(dba)3] or Pd(OAc)2 + PPh3
Palladium (II) can be used as a pre catalyst and be reduced in situ

2) More sophisticated ligands leave very high turn over frequency and turn over number under mild conditions

38
Q

How to promote cross coupling:

A

Strong sigma donating ligands are essential as they facilitate the oxidative addition step and help avoid catalyst deactivation by stabilising low coordinated palladium centres

Steric hindrance also has beneficial effects:

Promotes reductive elimination
Offers kinetic stability of low coordinated centres

39
Q

Hydrogen activation via oxidative addition

A

Either dihydride homolytic activation or monohydride heteolytic activation

Reduced pKa of H2 to between 0 and 20 from 35

40
Q

Agnostic interactions:

A

3c-2e bonding in electron deficient metals

Can be alpha or beta if hydrogens are available

Can lead to cyclometallation

41
Q

Characterisation of agostic interactions

A

1) H atoms not detected by X-ray crystallography so neutron diffraction must be used
2) proton NMR is often unclear and requires the solid state
3) IR has lower C-H stretching frequencies but they are hard to distinguish from other signals

42
Q

Characterisation of Metallicycles

A

1) H NMR

Signals at low frequencies (-5 to -45)
Non equivalent hydrides will couple
Coupling to phosphines can determine structure

2) IR M-H stretching frequencies in the range 1500-2200 cm^-1 but usually weak
3) clear X-ray distortions observed

43
Q

Why is functionalisation of C-H bonds a problem?

A

1) Alkanes are inert due to strong localised bonding
2) when alkanes react it is at high temperatures or with very reactive compounds, which is unselective
3) formed products are always more reactive than the alkane which can lead to overreaction

44
Q

C-H oxidative addition

A

Thermodynamically unfavourable due to strength of M-H vs M-C bonds

Products are prone to reductive elimination

Most activation is aromatic or involves agnostic interactions

45
Q

Cyclometallation and aryl ligands

A

Activation of ortho substituents is very easy

The outcomes are either

1) formation of a metal hydride with a higher oxidation state
2) cleavage of an anionic ligand

46
Q

Intermolecular Oxidative addition

A

1) C-H activation of arenes:

Good synthesis strategy of metal aryl complexes

2) can also allow the C-H activation of alkanes

47
Q

Sigma bond metathesis and C-H activation

A

Typical of early metals with a d0 configuration (as no OA can occur)

Concerted reaction via 4 centre TS

LM-R + M’-R’ to LM-R’ + M’-R

From d2-d10 both OA and SBM are permitted and often the mechanism can’t be confirmed

Can also be used for H2 and in rare cases for C-C bonds

48
Q

Metaloradical activation

A

Dimetallic complexes (typically Rh) exist in equilibrium with the monomeric complexes

These react with R-H to form two new complexes

Methane is the most active compound for this

Kinetic control- when toluene is used there is no aromatic C-H activation

49
Q

Electrophilic Activation:

The two different mechanisms

A

Can either proceed by:

1) ligand as internal base-

M coordinates to R-H bond then the bong breaks, with metal coordinating to R and XH. XH then dissociates

2) preactivation with external base

M has 2+ charge and coordinates to the RH bond, then an external base reacts with H leading to MR and HX IN ONE STEP

50
Q

Electrophilic activation:

A

Displacement of H by a metal:

[M+] + RH > [M]R + H

Typical of cationic complexes of strongly electrophilic metals in normal to high ox states (Pd(ll) Pt(ll) Pt(IV) Hg(ll) and Ti (III)

Often carried out in strongly polar media such as water or strong acids

Inorganic Chemistry (1 decks)

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